Light emitting diodes (LEDs) are ubiquitous to modem display technology. More than 30 billion chips are produced each year and new applications, such as automobile lights and traffic signals, continue to grow. Conventional diodes are made from inorganic compound semiconductors, typically AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue). These diodes emit monochromatic light of a frequency corresponding to the band gap of the compound semiconductor used in the device. Thus, conventional LEDs cannot emit white light, or indeed, light of any “mixed” color, which is composed of a mixture of frequencies. Further, producing an LED even of a particular desired “pure” single-frequency color can be difficult, since excellent control of semiconductor chemistry is required.
LEDs of mixed colors, and particularly white LEDs, have many potential applications. Consumers would prefer white light in many displays currently having red or green LEDs. White LEDs could be used as light sources with existing color filter technology to produce full color displays. Moreover, the use of white LEDs could lead to lower cost and simpler fabrication than red-green-blue LED technology. There is currently one technology for producing white LEDs, which combines a blue LED with a yellow phosphor to produce white light. However, color control is poor with this technology, since the colors of the LED and the phosphor cannot be varied. This technology also cannot be used to produce light of other mixed colors.
It has also been proposed to manufacture white or colored LEDs by combining various derivatives of photoluminescent polymers such as poly(phenylene vinylene) (PPVs). One device which has been proposed involves a PPV coating over a blue GaN LED, where the light from the LED stimulates emission in the characteristic color of the PPV, so that the observed light is composed of a mixture of the characteristic colors of the LED and the PPV. However, the maximum theoretical quantum yield for PPV-based devices is 25%, and the color control is often poor, since organic materials tend to fluoresce in rather wide spectra. Furthermore, PPVs are rather difficult to manufacture reliably, since they are degraded by light, oxygen, and water. Related approaches use blue GaN-based LEDs coated with a thin film of organic dyes, but efficiencies are low (see, for example, Guha, et al., J. Appl. Phys. 82(8):4126-4128, October 1997; III-Vs Review 10(1):4, 1997).
It has also been proposed to produce LEDs of varying colors by the use of quantum dots. Semiconductor nanocrystallites (quantum dots) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of quantum dots shift to the blue (higher energies) as the size of the dots gets smaller. It has been found that a CdSe quantum dot, for example, can emit light in any monochromatic, visible color, where the particular color characteristic of that dot is dependent only on its size.
Currently available light-emitting diodes and related devices which incorporate quantum dots use dots which have been grown epitaxially on a semiconductor layer. This fabrication technique is suitable for the production of infra-red LEDs, but LEDs in higher-energy colors have not been achieved by this method. Further, the processing costs of epitaxial growth by currently available methods (molecular beam epitaxy and chemical vapor deposition) are quite high. Colloidal production of dots is a much more inexpensive process, but these dots have generally been found to exhibit low quantum efficiencies, and thus have not previously been considered suitable for incorporation into light-emitting diodes.
A few proposals have been made for embedding colloidally produced quantum dots in an electrically conductive layer, in order to use the electroluminescence of these dots for an LED, but such devices require a transparent, electrically conductive host matrix, which severely limits the available materials for producing LEDs by this method. Available host matrix materials are often themselves light-emitting, which may limit the achievable colors using this method.